This invention relates to a screen suitable for wells such as wells of oil, water, natural gas, geothermal electric generation etc. and, more particularly, to a screen particularly useful as a deep well screen which is subject to extremely large external pressure, tension, compression and twisting forces.
A well screen used in a deep well of several hundred meters or even several thousand meters underground is required to have strength which is sufficient for resisting a large external pressure applied to the screen depending upon the structure and operation conditions of the well. As a screen satisfying such requiremnt, there has been widely used a pipe base screen which comprises a steel pipe formed with a number of round holes or slits, support rods arranged about the steel pipe in the axial direction of the pipe and a wire wound about the support rods. This pipe base screen however has the contradictory problem that, if the holes or slits are excessively formed in the steel pipe, strength of the screen against the external pressure becomes insufficient whereas, if the holes or slits are too few, the amount of take-in of fluid decreases with resulting deterioration in fluid collection efficiency. For securing sufficient strength in the use in a deep underground, the pipe base screen in some cases has to sacrifice the fluid collection efficiency.
For overcoming the problem of the pipe base screen, the applicant proposed a novel screen in Japanese Patent Publication No. 58-32275 (equivalent to U.S. Pat. No. 4,657,079). This screen comprises a reinforcing member of a generally cylindrical configuration, support rods arranged on the outside of the reinforcing member in the axial direction at a predetermined interval therebetween and a wire wound spirally on the outside of these support rods. In a section along a plain including the axis of the reinforcing member, slits continuous in the circumferential direction of the reinforcing member are formed with a predetermined pitch. The screen is capable of collecting fluid without using the pipe formed with holes or slits as in the prior art pipe base screen.
This screen disclosed in Japanese Patent Publication No. 58-32275 (hereinafter referred to as "ring base screen") having slits continuous in the circumferential direction of the reinforcing member has a higher rate of opening than the pipe base screen with resulting improvement in the fluid collection efficiency. Besides, the strength of the reinforcing member can be changed as desired by simply changing the shape of its cross section and its pitch and back-wash for preventing blocking of the slits by sand or the like is easy. The ring base screen, however, has the following problems in the manufacturing process and also in its strength under an extremely large external pressure.
In the ring base screen, the reinforcing member is formed by disposing a plurality of rings in parallel with an equal interval in the axial direction or winding a flat bar spirally. In each case, this reinforcing member is manufactured first and the support rods are arranged along the outer periphery of the reinforcing member and a wire is wound on the outside of the support rods by rotating the reinforcing member and the support rods together and the wire is welded on the support rods whereby a complete screen is obtained.
Since a well screen has a relatively large length compared to its diameter, the reinforcing member also is very long compared with its diameter. This necessitates insertion of a special jig inside of the reinforcing member during manufacture of the screen for provisionally holding the screen elements. Besides, this jig is used for transmitting power for rotating and axially feeding the reinforcing member and the support rods during winding of the wire, and this jig is essential in the manufacture of the ring base screen. This jig must be pulled off from the screen after completion of the manufacture thereof. Since, however, a strong constricting force is exerted over the entire length of the jig due to winding of the wire on the support rods, this jig must be of a special construction according to which the diameter of the jig can be reduced after completion of the screen. FIGS. 10 and 11 show a process of manufacturing the ring base screen employing such a special jig. A columnar jig 50 comprises three piece jig members 50a, 50b and 50c and a center rod 50d. The respective jig members 50a, 50b and 50c are displaceable in the radial direction by means of hydraulic jacks 50e which extend from the center rod 50d. A reinforcing member 51 is placed on this jig 50 and the jig 50 is pressed against the reinforcing member 51 by stretching the hydraulic jacks 50e. Then support rods 51 are arranged on the reinforcing member 51 and the jig 50, reinforcing member 51 and support rods 52 are fixed together by a chucking holder (not shown). Then a wire 53 is wound on the support rods 52 while imparting rotation and feed to the jig 50 and each contact point between the wire 53 and the support rods 52 is welded one after another by a welding electrode 54. After completion of the screen, the diameter of the jig 50 is reduced by withdrawing the hydraulic jacks 50e inwardly and then the jig 50 is pulled off from the screen.
The hydraulic jacks 50e are required in three places along the inner circumference of the jig 50 and the length of the jig normally is from six meters to twelve meters and, accordingly, several tens of hydraulic jacks 50e at the minimum are required in total. A hydraulic pump for applying a hydraulic pressure to the hydraulic jacks 50e must be disposed outside of the jig and hydraulic tubing must be provided from the hydraulic pump to the hydraulic jacks 50e through the inside of the jig 50. Since the jig 50 is rotating during the manufacture of the screen, the hydraulic tubing inside the jig 50 must be disconnected from the hydraulic pump during the manufacture of the screen and leakage of oil must be prevented by closing a cock of each tubing. After completion of the manufacture of the screen, the hydraulic tubing must be connected to the hydraulic pump to withdraw the hydraulic jacks for pulling out the jig 50 from the screen. Accordingly, a large number of hydraulic tubings and cocks are provided inside of the jig 50 and, besides, these must be housed in a structure adapted to the rotating jig. This requires a tremendous operation time and inevitably involves a high manufacturing cost.
The jig 50 has a limitation that it must have an inner diameter of more than about 150 mm due to the provision of the hydraulic jacks 50e and other attachments inside thereof and besides the manufacturing cost of the jig 50 is very high. Besides, the range of expansion and contraction of the jig 50 by the above described mechanism is rather limited so that jigs 50 of various diameters must be prepared in accordance with screens of various diameters with resulting increase in the manufacturing cost of the screen. Further, the operation of the jig 50 is very troublesome so that manufacture of the screen takes much time and labor.
In another aspect, the prior art ring base screen has strong resistance against an external pressure equivalent to the conventional pipe base screen when the reinforcing member 51 is held horizontally (i.e., vertically with respect to the support rods 52) as shown in FIG. 12 (a) but, when an excessive external pressure is applied to it, the reinforcing member 51 can no longer maintain its horizontal posture as shown in FIG. 12 (b) and resistance to the external pressure sharply decreases.
It is, therefore, an object of the invention to provide an improved ring base screen which can be manufactured easily and at a low manufacturing cost and can be constructed with an inner diameter of less than about 150 mm and which is further improved in strength.
It is another object of the invention to provide a method for manufacturing such improved ring base screen.